This invention relates to power production process system with motive gas heated by externally applied heat and expanded through a turboexpander to drive electrical generator and produce electricity or to produce useful shaft power, through a method having condition response control and ways of accommodating fluctuating parameters and process requirements, and in particular, as it pertains to such plants operating at a pressure reducing station of a gas transmission line.
The terms "gas expander" and "turboexpander" as used in the description of this invention, are intended to mean apparatus for gas expansion and conversion of kinetic energy to rotational shaft power, and are meant to be interpreted as one and the same.
Prior to natural gas being delivered to a customer from a high pressure gas pipeline, its pressure is reduced through a throttling valve to meet the distribution and utilization requirements. This throttling process of pressure reduction causes a lowering of the gas temperature as it approaches a constant enthalpy process. Depending on the range of pressure reduction, it is often required to add heat to the gas prior to expansion in order to maintain suitable temperature of the gas at a lower pressure and prevent formation of hydrates. For a pressure reduction of 3 to 1 there is approximately 15-20.degree. F. drop in temperature through a throttling valve. The gas is usually heated to an appropriate higher temperature by passing it through a hot water heat exchanger in which the water bath is heated by combustion of natural gas fuel, which due to the low throttling temperature drop and relatively uniform pressure reduction range is usually adequate.
When the gas is expanded through a turboexpander, the process approaches a constant entropy process which results in an isentropic temperature drop of about 120.congruent.140.degree. F. for a 3:1 pressure reduction with an actual design point temperature drop of about 70.congruent.90.degree. F. depending on isentropic efficiency of a turboexpander. The operation of the turboexpander at its design point produces maximum efficiency and minimum internal turboexpander throttling. The required temperature of the gas prior to expansion through a turboexpander is a function of the volume of gas entering, gas pressure upstream of expanding nozzles, gas exiting pressure or overall effective pressure ratio, turboexpander efficiency and the desired exit gas temperature. When the conditions of operation are variable with regard to the volume flow rate and/or entering gas pressure, and these conditions represent off-design point operation, then internal turboexpander throttling takes place prior to gas expansion through a nozzle, thereby reducing the effective pressure drop across an expanding nozzle and resulting in reduction of available energy for conersion to shaft power, lower process efficiency and higher exit gas temperature.
It is evident that increasing and maintaining the entering gas temperature to a fixed value determined by the requirements at the design point results in erratic exit gas temperature fluctuations at off-design points which result from variations of volume flow and pressure parameters.
Although a conventional gas heating process using water bath indirect heaters is adequate for simple throttling installations approaching constant enthalpy process, it is not considered a quick-acting temperature control system required for applications utilizing gas expanders which result in operation approaching a constant entropy process. Due to the greater effective temperature drop associated with use of gas expanders as means of pressure reduction and requirement for higher gas temperature prior to expansion than that of simple throttling, the conventional gas heating process previously described is considered to be inadequate. This is due to slow response characteristics of the water bath heater when heat transfer duty is varied and its limitations as to the maximum process gas temperature attainable.